This invention relates to an apparatus having means for producing and circulating hot air, intended particularly for shrinking a plastic film or the like around a support, such as a container. More particularly, the present invention concerns a novel apparatus for shrinking thermoplastic sheaths or sleeves around containers of different shapes and types.
It has been known for a long time to use films of thermoplastic material, such as polyvinyl chloride or the like as a means of packaging and displaying various articles, these films being capable of shrinking when exposed to the action of heat at a temperature close to the softening point of the material. Tunnel ovens of an appropriate length are used to cause shrinking, wherein the hot air, emerging from the ceiling, base or the side walls of the oven, is uniformly distributed over the entire length of the tunnel within which an endless conveyor carrying the articles to be packaged runs before moving to the outside of the oven. The purpose of this circulation of hot air is to condition the total surface of the packaging film in question, then to bring the film to the softening stage in order to release the stresses created when the film is stretched and to accomplish this as uniformly as possible.
The use more recently of sheaths or sleeves obtained by longitudinal joining of two contiguous edges of a heat-shrinkable film for the purpose of enclosing all or part of containers of various shapes, such as bottles, aerosol cans, cans of food and the like, thereby creating an ornamental strip, has posed a number of problems in the shrinking stage. In total or partial sheathing of the containers, the problem is not one of producing uniform distribution of the heat intended for shrinking, but one of creating zones wherein the air temperature is variable, for example increasing progressively, in order to cause selective shrinking of the film over the various parts, of more or less complex configuration, of the container to be sheathed. Hence, classical shrinking ovens are generally unsuited for this type of operation.
In one attempt to overcome this difficulty, a tunnel oven has been proposed, intended especially for shrinking of plastic rings or bands around the neck of containers, wherein movable side walls are provided, the side walls being movable lengthwise and/or vertically, so that the hot air, emerging from a slit disposed along these walls, can briefly touch the base of the ring or band and then, moving upward, the upper part of the ring or band when the container to be sheathed circulates continuously on a horizontal conveyor which traverses the oven. An oven of this type requires numerous elements intended to ensure the mobility of the walls, contributing to a relatively high cost and resulting in an apparatus of considerable complexity. Moreover, the control of the orientation of the walls, wherein heating elements are installed, is a delicate operation and one which must be carried out frequently for each type and/or size of container and sleeve.
To overcome the aforementioned disadvantages to a degree, a novel tunnel oven has been proposed wherein the hot air follows a selective path and is continuously recycled, the oven being provided with deflectors of a movable and adjustable type allowing selective and progressive direction of the hot air with respect to the various zones of the sleeve to be heat shrunk around a container. A tunnel oven of this kind, with a simple design and low cost, is completely satisfactory as far as effectiveness where directed to the application of sleeves to containers of complex shape is concerned. Nevertheless, use of a tunnel oven of this kind is generally limited to cases of partial or total coverage of containers whose height is not great, so that the circulation on the conveyor belt can be effected normally without the containers striking the upper part or roof of the oven. Of course, an oven with large dimensions can be provided, this oven being designed for objects of considerable height, but the effectiveness and efficiency of the selective direction of the hot air is then considerably and undesirably reduced.